The report, with two others this year from the UN-sponsored Intergovernmental Panel on Climate Change (IPCC) are considered to be the definitive distillations of humankind's understanding of human-driven climate change.

"The IPCC reflects the consensus of the vast majority of scientists in the field, and you can assess this by looking at the journals, the meetings, the conference proceedings, etc." says Gavin Schmidt, a climate scientist at the National Aeronautics and Space Administration's Goddard Institute for Space Studies, in an e-mail.

Yet a small but vocal minority continues to question the reports' conclusions. Because the IPCC is an organ of the United Nations, they say, the reports are politically skewed.

"We hear over and over the assertion that there is a consensus that 'global warming' is man-made and a crisis. Says who?" writes Joseph Bast, president of The Heartland Institute, a nonprofit dedicated to discovering "free-market solutions to social and economic problems," on its website.

Others say the authors are biased; dissent is quashed during the report's drafting, they charge. "Some of my comments and reviews were sort of rejected," says John Christy, a state climatologist at the University of Alabama at Huntsville and an IPCC contributing author who has doubts about humans' role in the observed warming. "I'm sure that [I] wasn't the only one."

The most vehement argue that evidence proving that human activity is causing global warming simply doesn't exist. "We've had a Greenhouse Theory with no evidence to support it – except a moderate warming turned into a scare by computer models," says S. Fred Singer, a professor emeritus of environmental science at the University of Virginia and vocal climate skeptic, in the press release for a study titled "Challenge to Scientific Consensus on Global Warming," published by the Hudson Institute.

In reply, IPCC authors point to what they characterize as the lengthy, exhaustive, and transparent process behind the reports.

'We can't ignore anything'

"The thing about the IPCC report which is not adequately appreciated by many is how rigorous and comprehensive the overall process is," says Kevin Trenberth, head of the National Center for Atmospheric Research's climate analysis section and a coordinating lead author on one chapter of the IPCC report. "We have to weigh all the evidence, but we can't ignore anything."

The IPCC itself does not conduct research but calls on a diverse group of scientists from around the world to review existing research . For example, Dr. Trenberth's group, Working Group I, had 152 lead authors. Some 25 percent were within a decade of having received their PhDs; 75 percent had not been a lead author on any previous IPCC report. And 35 percent hail from non-First World nations. The authors were nominated for participation by their own governments.

The scientist-authors pen a first draft of their findings, which is then sent to other experts in the field for review. Another draft is generated that receives comments from anyone who requests a copy. Review editors who operate independently of the authors ensure that every comment is logged and, if deemed relevant, responded to.

Yet another draft is generated and more reviewers, including governments, weigh in. Each country then solicits reviews nationally. In the United States, the government posted the draft on a website and invited comments from the public.

Some comments couldn't be included because they referred to research published too late to include, says Richard Somerville, a coordinating lead author in Working Group I, in an e-mail. "And we got a certain amount of pure nonsense. But if we rejected a comment, our reasons are written down and [are] public."

Working Group I received some 30,000 direct comments.

Conspiracy unlikely among so many

"If there was going to be a conspiracy among scientists, I can't imagine how you'd manage it," says Martin Manning, director of the IPCC Working Group I Support Unit. "Conspiracy theories run into credibility problems if you start to explore how it would work and how you'd have to make it happen."

Dr. Singer, also a skeptic, calls the full report "a very useful document." He reserves his stronger criticism for the Summary for Policymakers, a document all government delegations must unanimously approve.

"The summary is basically a political document," Singer argues. He points to wrangling over the Working Group I summary. China asked that the word "very" be removed from the text stating that observed warming was "very likely" due to human-emitted greenhouse gases.

In fact, the authors refused to change the wording. (A write-up of the Paris meeting is available at www.iisd.ca/climate/ipwg1/.) The authors and other delegates reminded China that their task was to draft a summary "for" not "by" policymakers. "The scientists determine what is said, but the governments determine how it's said," Trenberth says. "What we have to do is make sure it's consistent with the science."

As for a worldwide conspiracy, IPCC's Dr. Manning asks why 113 nations would endorse a fiction that's beneficial to none. He says he's never met a senior politician who is eager to deal with climate change. "A lot of politicians would breathe a huge sigh of relief if someone could actually say, 'No, we've got it wrong,' " he says.

Dissenting studies were weighed

But is dissent being quashed? Where the literature indicates a range of possibilities – the role of aerosols in climate change, for example – the full report discusses these issues at length. Even studies included in the just-released "Documented Doubts of Man-Made Global Warming Scares," a list by the Hudson Institute purporting to cast doubt on the consensus on human-driven climate change, were considered in the IPCC report.

"Those studies are taken into account," Trenberth says. "Every one of them."

The reason for the dearth of dissenting views may be quite mundane. According to one science historian, in the peer-reviewed scientific literature – the backbone of science and the source material for the IPCC report – hardly a dissenting voice is heard.

Naomi Oreskes, a professor of history at the University of California, San Diego, and an adjunct professor of geosciences at Scripps Institution of Oceanography there, searched for the keywords "global climate change" in a large database, a compilation of scientific journals. Of the 928 papers she found, not one questioned whether global warming was human-induced or if it was real. (The studies she found are a large representative sample, Dr. Oreskes says. She puts the total number of papers on global climate change at about 11,000.)

"The basic reality of anthropogenic global climate change is no longer a subject of scientific debate," she concludes.

Her study implies that since the IPCC must draw from scientific literature, it didn't find many papers that argued against human-driven change. Contrarian studies didn't make it through science's portal to respectability: scientific journals.

Maybe the scientific community does have it wrong about climate change, Oreskes says. After all, the majority has been incorrect before. But in this case, the contrarians are not, as they often paint themselves, on science's vanguard, she says. The discussion of global warming among scientists is not new – it's been going on for half a century, Oreskes says. The skeptics have had their day in court, she says: "It's just that nobody agrees with them."

Let us hear from you. Do you think climate-change skeptics are raising persuasive points or ignoring strong scientific evidence? Write us at:letters@csmonitor.com.

The first set of letters commenting on this series ranSept. 26. Another appears today.

What is the IPCC?

Founded in 1988 by the United Nations, the Intergovernmental Panel on Climate Change (IPCC) is charged with, among other things, identifying gaps in knowledge about climate change science and assessing the potential impacts of greenhouse gases. The IPCC has released four reports – in 1990, 1995, 2001, and 2007.

The IPCC does not conduct research. Instead, it enlists a diverse group of scientists from around the globe to review existing research. Governments nominate their own scientists for participation on the panel, and the result is a mix of contributors from all over the world. Some 600 authors took three years to draft the May IPCC report on "Climate Change 2007: the Physical Science Basis." More than 620 expert reviewers commented on its various drafts, and 113 countries approved it.

The IPCC has three working groups, each with a different area of focus. Each is charged with releasing its own report. In 2007 Working Group I focused on the physical science of climate change; Working Group II on impacts, adaptation, and vulnerability; and Working Group III on how to stop or slow down climate change (mitigation).

The IPCC will release a summarized report of all three working groups in November.

Each Working Group report has a summary for policymakers (SPM) that contains the key findings from the main report. Before its release, government delegations review the SPM line by line. All participating governments must approve the SPM unanimously. If a delegation absolutely disagrees with something in the report, then the line in question is excised. But governments cannot alter the text to better align with their own interests.

"We the scientists ... never lost control of the report," says Richard Somerville, a coordinating lead author for Working Group I, in an e-mail. "Governments wordsmithed it usefully ... but the science wasn't distorted or watered down in any manner. The SPM is a Summary FOR Policymakers, not BY Policymakers."